The present invention relates, in general, to short-pulse fiber lasers, and more particularly to femtosecond pulse fiber lasers operable at wavelengths of less than about 1.3 microns.
Femtosecond-pulse fiber lasers require anomalous dispersion wherein different wavelengths propagate at different speeds along the fiber, to compensate for the nonlinearities that most materials exhibit. Femtosecond erbium-doped fiber lasers operating at a wavelength of 1.55 μm can be constructed entirely of anomalous-GVD fiber to operate in the soliton regime, or with segments of normal-GVD and anomalous-GVD fiber to operate in the stretched-pulse regime. Ordinarily, however, dispersion in standard silica optical fibers is normal for wavelengths of less than ˜1.3 microns and, in contrast to the situation at 1.55 microns. Thus, it has not been possible to build a femtosecond laser that emits light at a wavelength of about 1 micron with standard fibers alone. Anomalous dispersion has been obtained in fiber lasers by the use of prism and grating pairs, but the unguided propagation of light through these bulk optical elements reduces the benefits of using the fiber medium.
There is great interest in the development of short-pulse fiber lasers at wavelengths below 1.3 μm, the zero-dispersion wavelength of standard silica fiber, but efforts in this direction have been hampered by the lack of a suitable anomalous-GVD fiber. In particular, ytterbium-doped fiber is attractive for high-energy, short-pulse operation, for pulses shorter than 50 fs and pulse energies up to 6 nJ can be generated with Yb fiber; however, all short-pulse Yb fiber lasers reported to date have employed prisms or diffraction gratings for anomalous GVD. Yb-doped amplifiers provide the highest pulse energies and average powers available from fiber-based sources, but these are all seeded by bulk oscillators or complicated multi-stage fiber sources with nonlinear wavelength conversions. For greatest utility it would be highly desirable to seed these amplifiers with an integrated fiber source.
Recently-developed microstructure fibers (which are also referred to in the literature as “holey fibers” and “photonic crystal fibers” (PCF)) can be designed to have a large anomalous waveguide dispersion which is a consequence of a small effective core area (the diameter is 1-2 microns) and large index contrast. The small area produces an effectively large nonlinearity, which can be advantageous or deleterious in femtosecond-pulse fiber lasers. Similar properties can be obtained by simply tapering an ordinary fiber to the same diameter; however, the length of the resulting taper is limited to ˜20 cm, and such tapered fibers are not commercially available.
It has been suggested that the anomalous dispersion in a PCF fiber could be exploited to construct modelocked lasers at wavelengths less than 1.3 microns; however, implementation of a femtosecond-pulse laser is far from obvious, owing to the properties of PCF. First, the large effective nonlinearity that inevitably accompanies anomalous dispersion is an issue, for although nonlinearity is essential for pulse formation, excessive nonlinearity also limits stable pulse formation in short-pulse fiber lasers. There is a window of stable pulse energies between these limits, but there is no guarantee that the window will be large enough to produce a practically-stable laser. A second issue is the fact that PCF fibers with anomalous dispersion are highly birefringent. This means that, in general, a pulse launched into such a PCF will be split into two components along orthogonal polarization axes. This splitting is undesirable because the two pulses may compete with each other, and thereby destabilize the laser. If the entire laser could be made of highly-birefringent fiber, it should be possible to avoid this splitting, for the pulse could propagate around the laser cavity as a single polarization component. However, highly-birefringent fiber doped with ytterbium or neodymium ions (which provide gain in the laser) is not commercially available. Accordingly, ordinary, low-birefringence fiber must be used, and it is the mixing of low- and high-birefringence that causes problems.
Although there has been a strong and clear motivation to develop all-fiber lasers at wavelengths other than 1.55 microns, the fact that there was no report of a femtosecond fiber laser with PCF for anomalous dispersion in the past four years is strong evidence of the difficulty of such an undertaking.